1. Field of the Invention
The present invention relates to an aqueous ink composition, an ink set, and an inkjet image forming method.
2. Description of the Related Art
In recent years, as demand for resource conservation, environment preservation, improvement in operational stability and the like has increased, use of aqueous coating materials and aqueous inks has been growing.
For example, UV-curable water-based inks have been known for a long time as a means for improving the fixability of a printed image obtained from a water-based ink.
As a water-soluble photoinitiator that is applicable to these UV-curable water-based inks, α-hydroxyketones, acylphosphine oxides and benzyl ketals are known.
A water-dispersed photoinitiator, in which an oil-soluble photoinitiator is dispersed in a water-based medium, is also known. For example, Japanese National Phase Publication No. 2003-512484 discloses an aqueous photoinitiator dispersion including an aqueous medium, a dispersant and an acylphosphine oxide photoinitiator. An aqueous photoinitiator dispersion of this kind is available from BASF Japan Inc. with a trade name of IRGACURE 819 DW. Further, for example, International Publication (WO) No. 2001/57145 discloses a photopolymerizable aqueous ink in which a monomer and a photoinitiator are included in oligomer particles being in an emulsion state.
However, in recent years, demand for improvement in speed and quality for printing has been increasing, and there is a strong need to impart sufficient fixability at a smaller amount of exposure, i.e., increase the sensitivity of a water-based photo initiator and achieve favorable dischargeability.
The present inventors have found that inks employing a known water-soluble photoinitiator do not exhibit sufficient curing sensitivity, and sufficient fixability cannot be obtained thereby. Furthermore, when a water-soluble group is introduced into an oil-soluble photoinitiator to form a water-soluble photoinitiator, there is a problem in that the molecular weight is increased due to the water-soluble group, and the number of moles of the photoinitiator contained in the ink is decreased, thereby lowering fixability.
Moreover, in the system in which the photoinitiator dispersion described in Japanese National Phase Publication No. 2003-512484 or the oligomer-containing ink described in WO 2001/57145 is applied to an inkjet ink, there is also a problem of insufficient dischargeability of ink, in addition to the problem concerning sensitivity. As such, it has been difficult to achieve both sufficient curing sensitivity (i.e., fixability) and sufficient dischargeability at the same time in inks employing known initiators.